1. Field of the Invention
This invention relates to a cardiac signal processing technique.
2. Description of the Related Art
Known methods of detecting heartbeats of a living body include a method of detecting body surface displacement with use of a piezoelectric sensor, a pneumatic sensor, an electromagnetic wave Doppler sensor, a ultrasonic wave Doppler sensor or the like. Also known is a method of detecting body surface movement occurring in response to beats of the heart by applying microwaves to a region around the chest of a subject to be measured (Patent Literature 1, Non-Patent Literature 1).
When using the method of measuring body surface displacement, the measurement is apt to be affected more significantly by noise caused by extraneous vibration or body motion of the subject himself/herself, because body surface displacement is relatively small. It is reported that when using the method of measuring body surface movement in a region around the chest, a respiratory signal with higher harmonics interfere with a cardiac signal (Non-Patent Literature 2). Non-Patent Literature 2 discloses that in order to suppress such interference, microwaves are transmitted by double side-band, and microwaves are transmitted to the back side of the chest.